Anti-inflammatory drugs and antibiotics are shown to decrease risk of breast cancer in women and yet the role of bacteria in etiopathogenesis of breast cancer is not well understood. Using mouse models of microbially-induced bowel disease, the authors show that gastrointestinal infection with Helicobacter hepaticus bacteria triggers tumor necrosis factor-α–mediated innate immune processes leading to mammary adenocarcinoma. The authors find that prior microbial challenge in these models reduces risk of tumorigenesis by enhancing regulatory T-cell competency, indicating that the interplay between gut bacteria and host inflammatory response modulates epithelial homeostasis and tumor development in distant organs such as the breast. These observations reveal pivotal roles for intestinal bacteria in breast cancer and raise interesting questions about links between human health and hygiene practices. For details, see the article by Rao et al. on page 7395 of this issue.